Process for treating iron and steel



Patented Sept. 2,1941

PROCESS FOR. TREATING IRON AND STEEL Earle C. Smith, Cleveland Heights,and George T.

' phorus,

Motok; Massillon, Ohio, assignors to Republic Steel Corporation,Cleveland, Ohio, a corporatlon= of New Jersey No Drawing. ApplicationMay 13, 1940,

Serial :No. 334,891

. -6 Claims.

This invention relates to the art of makin -irons and steels,'and isparticularly concerned with a new and improved process for making ironsand steels having new and unexpected properties and characteristics,including improved resistance to corrosion.

.The present invention is based on the discovery that nitrogen improvesth properties of irons and steels, including corrosion resisting ironsand steels. We have found that, contrary to generally accepted belief,nitrogen in the proper form and amount is not an element to be avoided,but in fact can be used to improve the properties and characteristics ofirons and steels of a wide variety ofrcompositions, including corrosionresisting irons having the fundamental composition of between about .01%and about .10% of carbon and between about .15% and about 1.5%. ofcopper, with or without small quantities of other elements, such asmolybdenum and mangane'se. Other elements whichmay be present in suchcompositions include-sulphur, phosphorus and silicon in the usual smallamounts. We have found that when an iron or this fundamental compositioncontains between about .005% and about .2% of nitrogen the resistance ofthe iron to corrosion is considerably-increased.

we have also discovered "that, contrary to previously generally acceptedbelief, ,'phos'phorus in proper amount is not an'element to be avoidedin corrosion resisting irons of the foregoing fundamental compositionbut, in fact, can be used to advantage to confer desirablecharacteristics thereon if and when present in about .04% and about 50%In practicing the present invention on irons and steels of the foregoingfundamental composition to improve the corrosion resisting propertiesthereof, the process may be carried out conveniently as follows: An ironor steel is made in the ordinary manner, as in an electric furnace, orin an acid or basic open hearth furnace, or in a Bessemer vessel'and/or-converter and may contain between about .15% and about 1.5%

amounts between of copper and between about 0l% and about .10% ofcarbon, with or without between about 95% and about 55% of molybdenumand with or without between about .01% and about 50% of manganese. Suchan iron or steel would orcontaining material is added to bring the totalphosphorus content of the melt to between about .04% and about 50%. Whenthe molten metal is poured from the ladle into the ingot molds, nitrogenbearing material may be thrown into the molds in an amount sufllcient toinsure the presence of between about .005% and about 20%- of nitrogen incold ingots. Alternatively the nitrogen containing material may be addedwhen the phosphorous is added, and both, or either, may be added to themolten metal in. the furnace, in the ladle or in the molds.

vCalcium cyanamid has been found to be a most suitable material for theforegoing purposes. The

addition of from about two to about five pounds.

of calcium cyanamid per ton of molten metal will carry a sufllclentamount of nltrogen'into the iron for the desired purposes.

In practicing the present process on compositions of the foregoinggeneral analysis for improved-resistance to corrosion, it is preferableto have ingredients present in the molten metal which will act asnitrogen fixers, i. e., elements which will combine readily withnitrogen liberated in the metal to form stable nitrides. Several metalshave this property including titanium,

vanadium, zirconium, columbium,, tantalum, silicon, aluminum, boron andberyllium. In the absence of nitrogen flxers the amount ofnitrogenretained in the. metal may vary with variations inthetemperature of themetal, the pouringrate of the metal, the oxygencontent-{oi the.

metal, the time-the metal is held before pouring, and the amount ofmechanical movement of the steel. When nitrogen fixers are present thesevariables do not materially afl'ect the amount r nitrogen retained bythe metal, for the nitrogen forms stable nitrides with the flxers andhence makes it pomible to control the amount oi nitrogen in the metalwithout the necessity of making any attempts to control those severalvariables.

.The process described in our former applica-- tion-Ber. No. 119,264inherently possesses many dinarily contain up to about .065% of micsvwhen the melt is tappedlnto a ladle a sumcient amount of phosphorus orphosphorus advantages. For example, when calcium cyanamid is added -tomolten metal it decomposes, liberating nitrogen in a nascent state andalso liberating th element or elements which were combined with thenitrogen in that compound. The nascent nitrogen so liberated is quiteeilec-' tive in removing oxygen dissolved in the molten metal, probablyby displacement of the oxygen by the nitrogen, and also seemsfto havesome eilect on combined oxygen. Such displaced oxygen and also free andcombined oxygen apparently comblneswith the element or elements releasedwhen the nitrogen containing compound decomposed, and the resultingoxide is removed from the metal, partly by the agitation or ebullitiontaking place in the molten metal when the nitrogen is thus liberated.The elements liberated when the nitrogen containing compound decomposes,also apparently combine with sulphur in the metal and the resultingsulphur compounds are carried out of the metal by the agitation thereof,thus reducing the sulphur conent of the metal,

We have discovered, as a result of the appreciation of the foregoinginherent results of the process as disclosed in our application Ser. No.119,264, that our process can be advantageously practiced, not only oncorrosion resisting irons and steels, but also on irons and steels of awide range of compositions, and with the result that the properties andcharacteristics of irons and steels so treated may be surprisinglyimproved. For example, by applying our process to molten irons-andsteels, we have been able to increase notch sensitivity andpermeability; to reduce their tendency toward caustic embrittlement,

characteristics obtained by our invention in large 1 part to the markedreduction in oxygen content and marked increase in nitrogen content ascompared with the amounts of those substances normally present in theuntreated metal, and in part to. the reduction of sulphur content of themetal.

A plain steel containing about .05% of carbon, and made by the mostadvanced practice with which we are familiar, normally contains betweenabout .06% and about .10% of oxygen, much of which is present in adissolved state. By means of our process we have been able to reducesuch an oxygen content to less than about .02% and in some instances toas low as about .005%. High carbon steels, for example those containingabout .50% of carbon, and alloy steels containing various amounts ofcarbon, chromium, nickel, and other alloying elements, when madeaccording to the most highly advanced practice with which we arefamiliar. normally contain from about .005% to about .08% of oxygen. Byour process we are able to eifect a marked decrease in the oxygencontent of these steels and to attain substantial improvements in theproperties and characteristics thereof, particularly in the respectabove mentioned.

Irons and steels, made according to the best present day practice withwhich we are familiar,

. normally contain oxygen contents, such as have been described above,and nitrogen contents which vary depending on the type of furnace orprocess used in making such irons or steels. For -example, in basic openhearth steel the nitrogen content is about 004%, in acid open hearthsteel it is about .008%, in basic electric furnace steel it is betweenabout .005% and .01%, in acid electric furnace steel it is about .0l%and in Bessemer steel it ranges from about .01% to about .03%.When'such. steels are treated by our process, the oxygen content isdecreased from a small amount where the oxygen content was normally verylow to as much as or more, in a plain low carbon steel where the oxygencontent was as high as .10% or higher.

At the same time the amount of nitrogen normally present in the steel isincreased by our process to the extent of from 40% or 50% to more thanseveral hundred per cent and simultaneously the marked improvement inphysical properties and characteristics of the metal so treated isobtained.

Additional improvements treaceable to our process include an increase inhardness of ferrite grains, particularly in the presence of aluminum andvanadium; an increase in toughness and particularly in low carbonsteels; an increase in their strength, hardness, ductility, elasticlimit,

the plasticity of thevmetal at rolling temperatures and a considerableextension of the hot working temperature ranges; and the development ofaustenitic and martensitic structures in low carbon compositions whichare analogous to structures present in compositions containing muchhigher amounts of carbon.

The present invention may be practiced by bringing intonitrogen-liberating contact with the molten iron or steel to be treatedwhen it is in a furnace, Bessemer vessel, or ladle, or while it is beingpoured into a mold, a quantity of a calcium cyanamid which is capable ofdecomposing in the molten metal and. liberating nascent or activenitrogen in -the metal in amounts sufflcient for the purpose desired.Cal-: cium cyanamid has-been found to be quite satisfactory. In contactwith the molten metal it decomposes into its constituent elements,libera'ting nascent nitrogen and calcium and canbon. The nitrogendisplaces oxygen dissolved in the metal and the calcium and carboncombine with free, combined, and displaced oxygen to form compoundswhich will be carried out of the metal partly by the ebullition oragitation of'the metal.

Furthermore, calcium cyanamid apparently aids in the desulphurization ofthe metal and 1 tends to break down non-metallic inclusions in themetal.

The amounts of calcium cyanamid which may be used advantageously incarrying out the present invention are rather critical and, accordingly,care should be taken to use such amounts as will not exert any harmfulefiect on the metal.

Generally speaking, the amount of calcium cyanamid used should rangebetween about pound and about 8 pounds per ton of metal. The amount ofsuch material which may be safely used depends partly upon the contentof carbon or carbide forming alloying elements present in the metal andpartly on the amounts of nitrogen flxers which are in the metal. When aniron or steel is to be treated by this invention and contains either alow carbon content, such as .10%, or a small amount of carbide formingalloying elements, and also contains small amounts of added nitrogenflxers, as much as 8 pounds of calcium cyanamid per ton of metal can besafely brought into the molten metal. As illustrative of this condition,reference is made to the two first above described plain irons or steelswhich are suitable for use for corrosion resisting purposes. I Thesecompositions contain up to about .10% or .15% of carbon, up to about1.5% of copper, up to about 25% of molybdenum and up to about .50% ofmanganese. In these compositions the carbon and carbide forming alloyingelements, and also the nitrogen ilxers, are all sufliciently low so thatwhen thecomposition is treated according to this process and with uptoas much as 8 pounds of calcium cyanamid, the oxygen content will bereduced,

the nitrogen content will be increased and the physical properties willbe improved, as has been described herein.

when the metal to be treated contains higher amountsof carbon, such as50%, or a somewhat lower carbon content coupled with carbide formingalloying elements, such as manganese, chromium, tungsten and molybdenum,the amount of calcium cyanamid should be decreased to avoid increase inbrittleness of the treated metal. For example, a steel containing about50% of carbon and about 1.50% of carbide forming alloying elements suchas manganese, chromium, molyb denum and tungsten may be improved asde-.

scribed herein provided the amount of calcium cyanamid brought into themetal does not exceed about 1 pound per ton of metal. When the contentof carbon or carbide forming alloying elements is-fairly low and thecontent of nitrogen fixers is fairly high,-the amount of calciumcyanamid which may be used may be somewhat higher than when the carbonor carbide forming alloying elements are high. For examplea steelcontaining about 20% of carbon, about .8% of carbide forming alloyingelements and about .25% of nitrogen fixers, including titanium,zirconium, vanadium, columbium, boron, beryllium, tantalum and aluminum,as much as about 6 pounds of calcium cyanamid per ton of metal may .be.safely employed and will produce the improvements-herein described.

In general, it may be said that with a plain low carbon steel containingsubstantially no carbide forming alloying elements and substantially noadded nitrogen fixers, up to about 6 pounds of calcium cyanamid may besafely brought into the metal and, when substantial amounts of fixersare present, up to about 8 pounds of calcium cyanamid may be similarlyused; with a plain low carbon steel containing substantial amounts ofcarbide forming alloying elements or a high carbon steelwith smallamounts of carbide forming alloying elements, up to about 2 pounds ofcalcium cyanamid may be similarly used, this amount being lowered as theamounts of carbon and carbide formers are increased, and, when example.-A fewp'oundsof one or more of such nitrogen flxers per ton'ofmolten-metal has been found to be suflicient to retain in the latterseveral hundred percent more nitrogen than would be retained by ourprocess if such fixers were not present.

We have discovered that our .invention is not limited to theintroduction of calcium cyanamid directly" into the molten metal, butthat the in- .vention may be advantageously carried out by the calciumcyanamid is added to or incorporated in such second slag. It is added toor incorporated' in the slag of the Bessemer or' open hearthfurnacepreferably as soon as the steel has begun to melt.;' In all suchinstances the calcium cyanamid may constitute as much as 25% by volumeof the slag, this amount'havlng been found to contain suflicientnitrogen to impart .the desired properties to the steel.

We believe that calcium cyanamid in the slag deoxidizes. theslag, which,in turn, deoxidizes the metal and that the nitrogen exerts itsbeneficial influence on the molten metal in much the same wayas itt-doeswhen it is added directly to the, molten metal; We believe that theseactions occur in much the same manner regardless of the typev of furnaceused.

The manner of practicing this form of our invention in'the various typesof non-electric fur naces will be understood from the following de- 5oxygen the metal itself contains a high amount of oxygen'and when theslag is low in oxygen the metal is likewise low in oxygen. Sometimesthis ratio is as high as 100 to 1.

vIn'the manufacture of irons and steels by the electric furnace'method,the second'slag which such a steel also contains substantial amounts-ofnitrogen fixers,- up to about 6 pounds of calcium cyanamid may besimilarly used.

We believe that the nitrogen liberated in,

These fixers may be added to the moltenmetal' before, or together with,the addition of the calcium cyanamid. Suitable nitrogen'fixers includemetals'such as copper, aluminum, manganese, molybdenum, 'arsenic,titanium, tantalum, vanadium, columbium, silicon, zirconium, boron andberyllium, and any one or more-of such metals may be used together. Suchnitrogen fixers may.

be introduced into the molten metal in metallic form ,as in the case ofcopper and aluminum for example, or in the form of a ferro-alloy as inthe case of aluminum, silicon and manganese for contains less oxygenthan the slag it replaced changes the previously existingphysico-chemical equilibrium and also lowers the ratio of oxygen for thehigher oxygen containing slag which it replaces exerts a metal. I whenthe oxy en content in electric furnace irons or steels is to be reducedto small amounts and this is to be accomplished by use of a second slag,the ingredients of the second slag are charged into the furnace and arethere converted deoxidizing effect on the molten into molten slag by theelectric arc. Often about.

three hours time is required for such melting. and before the slagbecomes effective. The heating is continued thereafter for about twohours or until the metal has been deoxidized.

We have discovered that if calcium cyanamid, as such, is added to thesubstances which are to 7 form the slag, much time can be saved; and the7 resulting metal will possess'improved properties and characteristics,both as compared with the prior practice. By adding such calciumcyanamid gen content and to bring about; the deoxidation of the metalthereby. The amount; of calcium cyanamid which may be so used may amountto as much as half of the total slag" rials. g Furthermore, we believethe; he calcium cyanamid, added as a part of .th second slag in anelectric furnace process, works into. the metal and decomposes therein,and its components act as has been described above, that is, the nascentnitrogen liberated by such decomposition displaces oxygen dissolved inthe metal and exposes such oxygen to the liberated calcium and carbon,and such oxygen, as well as some free and some combined oxygen in themetal, combines with the calcium and carbon and is taken into the slag.We also believe that the liberated calcium and carbon attack oxygen inthe slag and eliminate part of it as oxides of carbon, therebyreducingthe amountofoxygen in the slag and enabling the slag to reducefurther the amount of oxygen in the metal.

Certain of the above described features of our invention were disclosedin our application Ser. No. 119,264 flied January 6, 1937, .now PatentNo. 2,121,055 issued June 21, 1938, and also in our rining mate-'application Ser. No. 213,420 which was filed on June 13, 1938, andwhich was copending with application Ser. No. 119,264. The presentapplication is a continuation in part of our applications Ser. Nos.119,264 and 213,420, with the latter of which this application iscopending.

Having thus described our invention so that those skilled in the art maybe able to understand and practice thesame, we state that what wedesire-to secure by Letters Patent is defined in what is claimed.

What is claimed is:

1. In the manufacture of iron or steel, the method of decreasing theoxygen content, increasing the nitrogen content, and improving thephysical properties of the metal which includes the step of bringinginto nitrogen-liberating contact with the molten metal between aboutonehalf pound and eight pounds per ton of metal, of calcium cyanamid.

2. In the manufacture of iroh or steel, the method of decreasing theoxygen content, increasing the nitrogen content and improving thephysical properties of the metal which includes the step ofincorporating in the slag forming materials in the furnace a quantity,approximating 25% of the slag, of calcium cyanamid.

3. In the manufacture of iron or steel, the method of decreasing theoxygen content, increasing the nitrogen content and improving the'physical properties of the metal which includes the step of bringinginto the metal containing substantial amounts of carbon and carbideforming alloying elements and substantial amounts of nitrogen fixersbetween about one-half pound and about six pounds per ton of the metal,of calcium cyanamid.

4.- In the manufacture of iron or steel, the method of decreasing theoxygen content, increasing the nitrogen content and improving thephysical properties of the-metal which includes the step of bringinginto the metal containing a small amount of carbon and substantialamounts -of added nitrogen fixers about six pounds of calcium cyanamidper ton of the metal.

5. The method of making iron articles to be' exposed to corrodingconditions during use which includes the steps of making molten ironcontaining between about .01% and about .10% of carbon and ordinaryamounts of impurities such as sulphur and manganese, adding theretobetween about .15% and about 1.5% copper and between about .05% andabout 55% of molybdenum, and

bringing into the molten metal between about I pound and about 8 poundsof calcium cyanamid per ton of metal.

6. In the manufacture of iron or steel, the method of decreasing theoxygen content, increasing the nitrogen content, and improving thephysical properties of the metal which includes the step of bringinginto nitrogen-liberating contact with the molten metal a quantity ofcommercial calcium cyanamid containing between about .105 pound andabout 1.68 pounds oi nitrogen per ton of metal.

EARLE C. SMITH. GEORGE T. MOTOK.

